1. Field of the Invention
The present invention relates in general to electrical and electronic control systems for transmissions in engine-driven vehicles and power-transmitting apparatuses used in off-road vehicles, and in particular to electronic control systems for powershift transmissions.
2. Description of the Prior Art
In the past, power transmissions have been developed in which the transmission gear ratios are selected by electrical signals provided to solenoid valves. In large off-road vehicles, such as agricultural tractors, front-end loaders, road graders and the like, it is desirable to provide a large number of forward and reverse gear ratios. Due to the size of these transmissions, and the problems of mechanically linking operator-actuated controls to the transmissions, it is often preferred to select transmission gear ratios entirely by the operation of solenoid valves, as is done in a typical powershift transmission. The rate of clutch engagement in such transmission is very dependent upon hydraulic and mechanical controls such as orifices and one or more accumulators, which in general are not very effective for providing smooth or jolt-free gear shifts under all conditions. Even when such conventional engagement controls are set up for relatively smooth engagement of the powershift transmission in one application, it is difficult to tailor the clutch pressure modulation characteristics to other applications. Also, where relatively soft engagements are provided, this is often achieved by excessively prolonged engagement times or other undesirable characteristics. It may be possible to optimize a few gear shifts, for example, of the many possible gear shifts, but the remaining gear shifts may be either rather too fast, and therefore rough, or too slow.
In many transmission systems, clutch pedals are provided so that the operator may manually control the rate of clutch engagement. However, in off-road vehicles, such as those designed for certain agricultural or construction equipment applications, gear shifts occur frequently, and over the course of a day represent a significant source of operator fatigue. Also, in such manually-controlled transmissions, unnecessary torque overloads or excessive clutch wear results whenever the operator misjudges, or due to inexperience, inattentiveness or fatigue, is unable to provide the proper rate of clutch engagement.
The assignee of the present invention has been engaged in the development of microprocessor-based controllers for powershift transmissions for a number of years, and has developed electronic controllers which electrically actuate the various solenoid-operated hydraulic valves to select the desired transmission gear ratios in response to operator commands. For example, in U.S. Pat. No. 4,425,620 to Bachelor et al. entitled "Electronic Control for Power Shift Transmission," which is hereby incorporated by reference, there is disclosed a microprocessor-based electrical control system which has a mode select lever and a upshift/downshift pulser lever by which the operator may indicate the desired vehicle direction and gear in which the powershift transmission is to be operated. However, this control system does not provide an electronically controlled gradual engagement of the clutches of the powershift transmission during gear shifts, and simply operates the solenoids in an on-off manner. The patent does not address or recognize the possibility of directly controlling or modulating hydraulic pressures in the transmission so as to produce smooth, optimized clutch engagements during gear shifts.
The use of proportional actuator devices, such as hydraulic valves operated by torque motors is known in the agricultural and construction equipment art. Such proportional actuation devices are frequently operated by pulse width modulated (PWM) signals whose duty cycle is varied in proportion to the desired average or DC value desired to be produced by the actuator means. However, as far as we are presently aware, such proportional actuation devices have not been used on powershift transmissions before the present invention. This may in be due in part to the inherent problem associated with using such relatively delicate or sensitive equipment which must be finely controlled in the rugged and environmentally severe conditions to which a typical powershift transmission in an off-road vehicle is exposed. In the development of the present invention, applicants encountered a number of unexpected problems which had to be overcome to successfully apply the concept of utilizing a PWM solenoid valve as a proportional actuator device in a powershift transmission to obtain the controlled clutch engagements necessary to achieve smooth gear shifts under a wide variety of operating conditions.
Accordingly, a primary object of the present invention is to provide an electronic control system for a powershift transmission which utilizes a proportional actuation means, such as a proportional hydraulic valve operated by a solenoid supplied with an alternating electrical signal, such as a PWM signal, to automatically control the hydraulic system of the transmission to provide gradual clutch engagements required for smooth, efficient gear shifts.
Another important object of the invention is to provide an electronic control system which allows a number of key parameters to be quickly adjusted and stored in the memory of microprocessor means, so that the operation of proportional actuation devices associated with hydraulic valves within a powershift transmission can be readily tailored to fit almost any vehicle application to provide for quick and smooth clutch engagements.
Yet another object is to provide an electronic control system which can customize the clutch engagements in a powershift transmission for virtually all gear shifts.
Still another object of the present invention is to provide a control system which automatically compensates for a number of variables which would otherwise detrimentally influence the quality of clutch engagements in a powershift transmission, including changes in temperature of the transmission and solenoids and in the magnetic coupling between adjacent solenoid coils of the solenoid-operated hydraulic valves.
One more object of the present invention is to eliminate the need to provide a separate voltage regulation supply to feed electrical power to a proportional solenoid used in an off-road vehicle, by providing for automatic adjustment of the duty cycle of the PWM signal driving the proportional solenoid which counteracts changes in or drifting of the nominal voltage in the vehicle's electrical supply system.